THERMOLYSIS OF CP-STAR-RH((ETA-4-ETA-1-C4ME4S)FE(CO)4) - A CASE-STUDY IN THIOPHENE DESULFURIZATION

Cp*Rh(eta-4-C4Me4S) (1) and Fe3(CO)12 react to give the ferrole Cp*Rh{eta-5-C4Me4Fe(CO)3} (3) together with trace amounts of Cp*Rh{eta-4:eta-1-C4Me4S)Fe(CO)4} (2). Compound 2 was prepared in high yield by the reaction of 1 with Fe(CO)5 in the presence of Me3NO. Thermolysis of 2 in refluxing toluene gave 3, free C4Me4S (tetramethylthiophene, TMT), and (Cp*Rh)2(mu-CO)(mu-3-S)Fe(CO)3 (4) in ca. 2:2:1 molar ratio. The efficiency of the conversion of 2 to 3 increased with added Fe3(CO)12 concomitant with the diminution of the yield for 4. Control experiments showed that 3 and 4 are stable to Fe3(CO)12 in refluxing toluene. Furthermore, by labeling both the rhodium, with C5Me4Et, and the thiophene, as C4Me4S-3,4-d2, we showed that the Rh-C4Me4S moiety remains intact during its desulfurization. The structures of 2-4 were determined by single-crystal X-ray diffraction. The desulfurization process illustrates the following mechanistic points: (i) transition metals play a dual role in thiophene desulfurization by separately stabilizing the desulfurized hydrocarbon and accepting the sulfur, (ii) the hydrocarbon is stabilized in the form of a metallacycle that structurally resembles thiophene, and (iii) the hydrocarbon and sulfide are stabilized in heterometallic environments.